Exercise devices are known in which exercise causes rotary motion of a member, with the rotary motion being opposed by various braking mechanisms. Typical of rowing or bicycling apparatus is a friction brake which applies a frictional retarding force to a fly wheel. One of the major problems with such a braking system is the so called break away torque necessary to start the fly wheel in motion at the beginning of the exercise. Note, an unusual amount of user force is necessary in order to overcome this break away torque, which makes exercise uncomfortable. Typical friction braking devices are described in U.S. Pat. Nos. 1,974,445; 2,725,231; and 2,512,911. Friction brake devices are also described in the following publication: "A constant-torque brake for use in bicycle and other ergometers," J. Y. Harrison J. App. Phys. Vol. 23, No. 6, Dec. 1967.
Electromagnetic braking systems have also been utilized in exercise equipment, the most common of which being an alternator which provides a retarding force against which the user exercises. Such devices are illustrated by U.S. Pat. Nos. 857,447; 3,442,131; 3,555,326; 4,060,239; 4,082,267 and 4,084,810. Other brakes for exercise apparatus are shown in U.S. Pat. Nos. 625,905; 683,124; 782,010, 783,769; 1,239,077; 3,497,215; 3,558,130; 3,586,322; 3,592,466; 3,711,812; 3,765,245; 3,962,595; 4,047,715; 4,085,344; 4,112,928; 4,130,014; 4,298,893; 4,347,993; 4,350,913; 4,396,188; 4,416,293; 4,512,566; 4,687,195; 4,708,338; and 4,798,378. Various foreign patents showing exercise equipment include SU 869,781; DT 2,830-691; GER 743,133; IT 468,973; SW 7706-583; SU 371,950; and DEN. 83817.
Of particular interest are ferromagnetic eddy current type braking systems in which the pole faces of the electromagnets are placed outside a ferromagnetic rim of the fly wheel employed. One of the major problems with such a device is the break away torque due to residual magnetism. Moreover, due to the placement of the electromagnetic pole faces outside the fly wheel when the fly wheel is heated due to the braking process, the wheel expands and binds against the pole pieces. An additional problem with such expansion is that the expansion is in a direction which varies the gap between the rim of the fly wheel and the pole piece. The result is that due to thermal expansion, an increasing torque is applied, with the relationship between the expansion and the additional torque being non-linear. Such a ferrous metal eddy current brake is shown in U.S. Pat. No. 4,798,378 in which a ferrous rim is placed opposite a stationary electromagnet.
By way of further background, as illustrated in an article entitled "A Bicycle Ergometer with Electric Brake," by Frances G. Benedict and Walter C. Cady in the Carnegie Institution of Washington Journal in 1912, a bicycle ergometer is proposed in which a copper disc is positioned between the pole pieces of electromagnets with the pole pieces being on diametrically opposite sides of the copper disc. While the system described by Benedict et al. produces an eddy current braking system which is effective in producing a retarding torque, the utilization of copper presents a number of problems.
Perhaps the first and most important problem is that the copper warps during usage due to thermal expansion characteristics and due to its inherent ductility. The problem then becomes maintaining the spacing between the opposed pole pieces so as to provide a regulatable constant torque during the period of exercise. It will be appreciated that the provision of a constant torque for a constant setting dialed in by the user is important because during the period of exercise which may last as long as an hour or two, the physical characteristics of the braking system normally change due to thermal expansion of the mechanical parts. The result is neither proper calibration nor comfort for the user of the exercise device, due to constant adjustments which must be made in order to maintain constant torque.
Thermal considerations aside, variation in torque with speed of exercise is unacceptable. Prior problems in the variation of torque with speed are described in the following articles: C. Lanooy & F. H. Bonjer, "A Hyperbolic Ergometer For Cycling & Cranking", J. Appl. Physiol. vol. 9, pp. 499-500, 1956, in which a copper disc was utilized in an eddy current braking system, and A. Krogh, "A Bicycle Ergometer and Respiration Apparatus For The Experimental Study of Muscular Work", Skand. Arch. Physiol. 33, pp. 375-394, 1913, in which work per revolution is said to vary with speed of the copper disc.
Thus, it is a design goal to achieve constant torque over a wide range of rotary speeds of the disc. Additionally, it is also important that the torque be constant throughout the period of exercise. Copper, while being an extremely good electrical conductor, has a problem that the torque delivered by the system employing the copper disc is neither relatively flat or constant for the range of exercise intended; nor is the torque provided by the eddy current/copper disc system controllable without elaborate feedback systems. Thus, for instance, the response of such a system to variations in pedal rotation of between 40 and 100 rpm is that, for a constant setting, the retarding torque is highly dependent upon the rotary speed of the pedals. The result for the end user is that there is an extremely annoying difference in the retarding force when pedaling at different speeds.
The variability of the retarding torque is more troublesome in medical applications when it is important that a constant torque be presented to the user of the exercise device in order to obtain proper measurement of exercise activity.